Automatic gain control in a zero intermediate frequency radio device

Abstract

A zero intermediate frequency radio device has an antenna for receiving a radio frequency signal, and a frequency down converter for down converting the received radio frequency signal to a zero intermediate frequency signal. The radio device further has controllable amplifier stages, controllable AC-coupling stages, controllable filter stages, and a received signal strength indicator. A signal resolving range of the received signal strength indicator is below a high dynamic range of the received radio frequency signal. The radio device further has an automatic gain controller. The automatic gain controller initially sets the gain of the Rx path of the radio device to a maximum of minimum gain, and then waits for DC-offsets in the Rx path to cancel. If, at maximum or minimum gain, a reading of the received signal strength indicator is within a particular range, the automatic gain controller sets the gain to the reading. Then, automatic gain control settles. If this is not the case, the gain of the Rx path is stepwise decreased or increased, while repeating the DC-offset cancellation step of waiting for the DC-offset to cancel, until the reading of the received signal strength indicator falls in the signal resolving range around the then set gain of the Rx path.

What is claimed is:  
     
       1. A zero intermediate frequency radio device comprising: 
       an antenna for receiving a radio frequency signal, said radio frequency signal exhibiting a high dynamic range;  
       a frequency down converter for down converting said radio frequency signal to a zero intermediate frequency signal, said frequency down converter comprising a mixer, an AC-coupler, and a received signal strength indicator with a signal resolving range that is below said high dynamic range, said AC-coupler being coupled to an output of said mixer;  
       a signal processor for processing said zero intermediate frequency signal;  
       at least one amplifier coupled between said antenna and said signal processor; and  
       an automatic gain controller for at least gain controlling said at least one amplifier,  
       said automatic gain controller being configured to set a gain of said at least one amplifier by setting said gain to a predetermined gain, by waiting a predetermined time for allowing DC-offset signals in said radio device to decay, by checking whether a reading of said received signal strength indicator is within said signal resolving range, and by setting said gain in accordance with said reading if said reading is within said signal resolving range.  
     
     
       2. A zero intermediate frequency radio device as claimed in  claim 1 , wherein said predetermined gain initially is at a maximum gain value of said gain, and said automatic gain controller is configured to stepwise decrease said gain, while repeatedly waiting said predetermined time and repeatedly performing said checking until said reading is within said signal resolving range. 
     
     
       3. A zero intermediate frequency radio device as claimed in  claim 1 , wherein said predetermined gain initially is at a minimum gain value of said gain, and said automatic gain controller is configured to stepwise increase said gain, while repeatedly waiting said predetermined time and repeatedly performing said checking until said reading is within said signal resolving range. 
     
     
       4. A zero intermediate frequency radio device as claimed in  claim 2 , wherein said at least one amplifier is comprised of a radio frequency amplifier coupled between said antenna and an input of said mixer, and of a zero intermediate frequency amplifier coupled between an output of said mixer and said signal processor, and said gain is a product of a first gain of said radio frequency amplifier and a second gain of said zero intermediate frequency amplifier, said automatic gain controller being configured to first modify said first gain when setting said gain. 
     
     
       5. A zero intermediate frequency radio device as claimed in  claim 3 , wherein said at least one amplifier is comprised of a radio frequency amplifier coupled between said antenna and an input of said mixer, and of a zero intermediate frequency amplifier coupled between an output of said mixer and said signal processor, and said gain is a product of a first gain of said radio frequency amplifier and a second gain of said zero intermediate frequency amplifier, said automatic gain controller being configured to last modify said first gain when setting said gain. 
     
     
       6. A zero intermediate frequency radio device as claimed in  claim 4 , wherein after modifying said first gain, said waiting said predetermined time is a first predetermined time. 
     
     
       7. A zero intermediate frequency radio device as claimed in  claim 6 , wherein said automatic gain controller is configured to modify said second gain, and said waiting said predetermined time is a second predetermined time, said second predetermined time being substantially shorter than said first predetermined time. 
     
     
       8. A zero intermediate frequency radio device as claimed in  claim 1 , wherein after settling of said gain, said automatic gain controller is configured to reduce a cut-off frequency of said AC-coupler. 
     
     
       9. A zero intermediate frequency radio device as claimed in  claim 8 , wherein said automatic gain controller is configured to reduce said cut-off frequency to such an extent that AC-coupling effectively becomes DC-coupling. 
     
     
       10. A zero intermediate frequency radio device as claimed in  claim 1 , wherein at an output side of said mixer said frequency down converter comprises a channel filter, and said automatic gain controller is configured to distribute gain control over said mixer, said AC-coupler, said at least one amplifier, and said channel filter. 
     
     
       11. A zero intermediate frequency radio device as claimed in  claim 1 , wherein said frequency down converter is a quadrature converter. 
     
     
       12. A zero intermediate frequency radio device as claimed in  claim 1 , further comprising a transmit branch and a receive-transmit switch, said antenna, said frequency down converter, and said transmit branch being coupled to said receive-transmit switch. 
     
     
       13. A zero intermediate frequency radio device as claimed in  claim 1 , wherein, after said automatic gain controller has set said gain, gain to an input of said received signal strength indicator is reduced by a predetermined amount in order to detect a decrease in signal level. 
     
     
       14. A gain control method for gain controlling of a zero intermediate frequency radio device, said zero intermediate radio device comprising an antenna for receiving a radio frequency signal, said radio frequency signal exhibiting a high dynamic range, a frequency down converter for down converting said radio frequency signal to a zero intermediate frequency signal, said frequency down converter comprising a mixer, at least one amplifier, and an AC-coupler, said AC-coupler being coupled to an output of said mixer, and said zero intermediate frequency radio device further comprising a received signal strength indicator with a signal resolving range that is below said high dynamic range, said method comprising at least setting a gain of said at least one amplifier by: 
       setting said gain to a predetermined gain;  
       waiting a predetermined time for allowing DC-offset signals in said radio device to decay;  
       checking whether a reading of said received signal strength indicator is within said signal resolving range; and  
       setting said gain in accordance with said reading if said reading is within said signal resolving range.